The present invention relates to a semiconductor device and, more particularly, to a semiconductor device for driving a spindle motor of a hard disk apparatus utilizing back electromotive force of a motor. The present invention also relates to a motor driver circuit using the above-mentioned semiconductor device.
FIG. 1 is a schematic longitudinal cross-sectional view of a conventional output field effect transistor, in which reference numeral 1 denotes a well base area, 2 a field oxide film, 3 a channel stopper area, 4 a high density base area, 5 a base area, 6 a source area, 7 a high density drain area, 8 a drain area, 9 a source electrode, 10 a drain electrode, 11 a gate polysilicon electrode, 12 a interlayer insulating film and 13 a semiconductor substrate.
In the field effect transistor as above, since the source electrode 9 contacts both the source area 6 and the high density base area 4, a parasitic bipolar transistor formed with the source area 7 (emitter) and the base area 5 (base) and the well base area 1 (collector) is prevented from operating, which is, however, undesirable because the current density is increased. Further, if the voltage applied between the drain and the source is increased, the parasitic transistor can be turned on.
In order to prevent such an undesirable phenomenon, as shown in, for example, JPA 56-7347, an attempt has been made to increase the carrier density in the base area other than the channel area to make the parasitic transistor difficult to turn on.
Such a field effect transistor has been widely used in the driver circuit for driving the motor because the current loss can be made small as compared with the ordinary output bipolar transistor.
As such driver circuit, a circuit as shown in FIG. 2 has been widely used. In FIG. 2, the driver circuit comprises a first portion provided between the spindle motor 42 and a control circuit 20A for controlling the same and a second portion provided between a voice coil motor 45 and a control circuit 20B for controlling the same. The first portion is connected to the output end of the control circuit 20A via the input terminals 27 through 32 of the spindle motor and also to the spindle motor 42 via the input terminals 39 through 41 of the spindle motor. Numeral 21 denotes the input terminal of the control circuit 20A.
On the other hand, the second portion of the driver circuit is connected to the output end of the control circuit 20B via the input terminals 33 through 38 of the voice coil motor, and also to the voice coil motor 45 through the input terminals 53, 54 of the voice coil motor. Incidentally, numeral 22 denotes the input terminal of the control circuit 20B, 23 a power supply terminal and 26 a grounded terminal.
The first portion has a bridge circuit formed with two sets of MOS transistors 46, 47, 48 and 49, 50, 51, and the drain of the first set of MOS transistors 46, 47, 48 is connected in common to the supply terminal 24 for the spindle motor through a diode 58, and the source is connected to the input terminals 39 through 41 of the spindle motor respectively. On the other hand, the source of the second set of MOS transistors 49, 50, 51 is connected in common to the grounding terminal 26, and the drain is connected to the input terminals 39 through 41 of the spindle motor respectively.
Similarly, the second portion also has a bridge circuit formed with two sets of MOS transistors 62, 63, 64 and 65, 66, 67, and the drain of the first set of MOS transistors 62, 63, is connected in common to the power supply terminal 25 for the voice coil motor. The source of the MOS transistors 62, 64 is connected to the input terminal 53 of the voice coil motor and the source of the MOS transistor 63 is connected to the input terminal 54 of the voice coil motor. On the other hand, the source of the second set of MOS transistors 65, 66, 67 is connected in common to the grounding terminal 26. Further, the drain of the MOS transistor 64 is connected to the power supply terminal 24 for the voice coil motor via the foregoing diode 58. Sign 22 denotes the input terminal of the voice coil motor control circuit 20B.
In the hard disk apparatus, when the power supply voltage of the spindle motor is reduced, it has been generally practiced that the voice coil motor is returned (retracted) to the starting position by utilizing the back electromotive force of the spindle motor. To this end, it is indispensable to provide a diode 58 for preventing reverse current for the conventional driver circuit utilizing the MOS transistor in order to prevent the reverse current of the back electromotive power caused by the parasitic diode of the MOS transistor to the power supply.
To be more specific, if a conventional semiconductor device shown in FIG. 1 is used in a three-phase driver circuit of FIG. 2, which drives the spindle motor of the hard disk apparatus and which allows the back electromotive force of the motor to be utilized, the following problems will occur. That is, according to the conventional semiconductor device, since the parasitic diode is formed between the drain and the source of the MOS transistor, the back electromotive force generating through rotation of the motor flows toward the power supply through the parasitic diode formed to the transistors 46 through 48, 64 connected to the power supply side. In order to prevent this reverse current, it becomes necessary to provide a diode 58.
However, this insertion of the diode causes voltage drop. For example, if the power supply voltage is 3 V, then the voltage drop on the order of 0.5 V will occur between both ends of the inserted diode. As a result, a necessary voltage could not be applied to the motor.
Accordingly, an object of the present invention is to provide a semiconductor device improved so that the formation of the foregoing parasitic diode can be effectively avoided.
Another object of the present invention is to provide a motor driver circuit adapted for the hard disk apparatus which is provided with the improved semiconductor device.